BASTERIA, Vol. 61: 89-98, 1998

Rissoa membranacea (J. Adams, 1800) (, Prosobranchia) from the Dutch Wadden Sea

Gerhard+C. Cadée

Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands

Rissoa membranacea disappeared from the Dutch Wadden Sea together with the large subtidal

stands L. ofeelgrass Zostera marina in the early 1930s, due to a still not completely understood

‘wasting disease’ in Zostera. A small population of R. membranacea survived together with Zostera

marina in inland Bol’ 1981. shells of R. an brackish water ‘de on Texel until Empty membranacea

can still be found at these localities. The relatively large size of the top- ofWadden Sea

R. membranacea indicates that the larvae In this were lecithotrophic. respect they arecomparable

to R. membranacea type B from the Roskilde Fjord of Rehfeldt (1968) and Warén (1996) and

R. membranacea to s.s. as described by Verduin (1982b). It is, however, questionablewhether R.

membranacea Type A ofRehfeldt and R. labiosa sensu Verduin can be separated from R. membranacea

s.s. They differ only in the smaller size of the embryonic whorl indicating planktothrophic

larvae. This well indicate different oflarval pelagic might as two types development,lecithotrophic

andplanktotrophic in one and the same R. membranacea s.l. Apical dimensions in samples

from the Ria de Arosa (Spain) and from Dutch Eemian deposits show a gradual change from

small ‘R. labiosa’ to larger ‘R. membranacea’ values, which supports the merging ofboth nominal

species and corroborates Warén’s suggestion of a continuum in larval development in R.

membranacea between lecithotrophyand planktotrophy as well as newer ideas of a gradually and

easily crossed transition between lecithotrophy and planktotrophy of McEdward (1997).

Key words: Gastropoda, Prosobranchia. , Rissoa membranacea, larval development,

lecithotrophy, planktotrophy, Wadden Sea, Netherlands.

INTRODUCTION

Rissoa membranacea J. Adams, 1800) is a small prosobranch gastropod, occurring from

Norway and the Baltic near Rugen to the Canary Islands, and probably also in the Mediterranean. It lives associated with marina L.. It is normally eelgrass Zostera an

annual species which breeds in summer in the northern part of its distribution (Fretter

& Graham, 1978). Egg capsules are deposited preferably on Zostera leaves (Van Goor,

1919; Lebour, 1934; Smidt, 1938). After hatching they grow rapidly. Van Goor (1919)

observed 1 mm shells in July and in September they had reached already their max-

imum size of 5.5 - 9 mm. Smidt (1938) found an equally rapid growth in the Sound

from 1 3.5 - 4 their maximum size for this in mm early August to mm, locality, early fall September. During winter Zostera leaves off and the greater part ofthe R. membrana- small attains the in the cea population perishes, only a part breeding stage following former summer (Van Goor, 1919; Smidt, 1938). In the Zuiderzee R. membranacea was Benthem The very common (Van Goor, 1919;Van Jutting, 1933). species was badly affected by the Zostera disease in NW. Europe in the 1930s. Waren (1996) presents an of its and up-to-date account ecology. 90 BASTERIA, Vol. 61, No. 4-6, 1998

The variable shell ofR. membranaceahas induced highly morphology long ago already Schwartz authors to a division in differentspecies and varieties (e.g. von Mohrenstern,

1864). Verduin (1982a,b) meticulously measured numerous Recent shells of R. mem- from Black Sea and the Mediterranean branacea s.l. from European seas ranging the to northern Europe and the Baltic. He was able to distinguish two variable 'species', differed in the size of the whorl. The forms with the smaller which only apical he identified R. labiosa those with R. membranacea as (Montagu, 1803), a larger apex as Verduin mentioned Rehfeldt who also had found in R. s.s. (1982b) (1968), two types membranacea. She able relate these difference in larval was to types to a development: with slender and larvae in B with pelagic larvae in type A the apex, non-pelagic type the Rehfeldt's results illustrated the of Thorson larger apex. nicely 'apex theory'

(1950), which states that in general a clumsy large apex in gastropods points to non- pelagic development and lecithotrophic larvae (feeding on yolk and not requiring planktonic food), while a narrowly twisted apex points to a pelagic development and larvae. Since Thorson's seminal facul- planktotrophic papers (Thorson, 1946, 1950)

tative planktonic feeding has been observed in lecithotrophic larvae e.g. in the nudibranch Phestilla sibogae Bergh, 1905 by Kempf & Hadfield (1985) and Miller (1993)

indicating a gradual transition between lecithotrophy and planktotrophy. McEdward and reversible transition between both (1997) suggests a gradual easily feeding types. Also so-called adelphophagy (Bouchet, 1989), the ingestion by developing larvae of

nurse eggs or embryo's in the same egg-capsule, has been observed and complicates the picture.

Waren (1996) studied > 1,000,000 R. membranacea specimens from all parts of the distributional in and confirms the existence of Rehfeldt's A area western Europe type and B, giving excellent SEM pictures of the difference in the top-whorl. Waren found

Rehfeldt's A = Verduin's R. labiosa from northern type (with planktotrophic larvae)

Europe to southern Portugal (and presumably in the Mediterranean), whereas Reh-

feldt's B = Verduin's R. membranacea in the type s.s. (with development egg capsule,

veligers feeding on nurse eggs) is known only from Roskilde fjord, south-west France and the United But he also that the situation be Kingdom. reports may more com-

plicated: planktotrophic larvae have been found to consume undeveloped eggs before which enables them size in the and hatching, to grow to a larger egg-capsule probably

abbreviate their pelagic period. Waren suggests a continuum between lecithotrophy conditions. and planktotrophy as an adaptation to local

Thorson (1950) observed a clear N-S trend in larval development throughout the

North Adantic, later named 'Thorson's rule'. He observed only prosobranchs with non-

pelagic larvae in East Greenland, to the majority (68%) with pelagic and less (32%) Islands. studies since with non-pelagic larvae at the Canary Numerous 1950 support

a general increase in non-pelagic, brooded development with increasing latitude, al-

though exceptions have been found in the Antarctic and deep-sea invertebrates (see

Levin & for Verduin did not Rehfeldt's Bridge, 1995, a review). (1982b), integrate results in his did he mention how his data fit in with the paper, nor nicely apex theory

of Thorson: Verduin observed R. labiosa with the slender and therefore most apex probably with pelagic planktotrophic larvae, from the Mediterranean to Denmark. His

R. membranacea with the and larvae did large apex probably lecithotrophic occur more to the north, in the Baltic, the North Sea, the Irish Sea and the Channel, but not south

of Bretagne. There is an area of overlap of the two 'species' in which among others The Netherlands but Verduin did material from are situated, unfortunately not study

there. This leaves the possibility that both his R. membranacea and R. labiosa once lived Cadee: Rissoa membranaceafrom the Dutch Wadden Sea 91

in The De Boer De Netherlands, as suggested e.g. by & Bruyne (1991) and De Bruyne (1994). R. membranacea s.l. lives in the Dutch no longer Wadden Sea (De Boer & De Bruyne,

1991) and is also extinct in the German and Danish Wadden Sea (Von Nordheim et

al., 1996). With the disappearance of large sublittoral stands of Zostera marina in the

early 1930s (Polderman & Den Hartog, 1975), R. membranacea and another small gastropod Lacuna vincta living on eelgrass, (Montagu, 1803) (synonym L. divaricata (Fabricius, 1780))

disappeared as well. Van Benthem still mentioned both Jutting (1933) species as very abundant in eelgrass fields, which fauna was well studied by van Goor (1919), but the

species had probably already disappeared at the time of publication in 1933. On the other its in Wadden Sea hand, occurrence the was not discovered before the beginning of this century. The first specimens found of R. membranacea date from 1909, collected W.C.Van Heurn Texel and from by on (Vernhout, 1912) 1912 on Griend, collected Van der Sleen In the Wadden Sea shells of both by (1913). now only empty species be found and in small numbers observations and R. can only (personal pers. comm. Also, the small of R. membranacea, which survived Dekker, NIOZ). population among marina in brackish inland 'de Bol' Zostera a water on Texel, has disappeared. 'De Bol'

had a connection with the Wadden Sea via an ebb-sluice which opened automatically low tide because then the of the the landside became during pressure water on higher

than that from the Wadden Sea water. R. membranacea was discovered here in the 1950s

(Swennen, 1955) and repeatedly mentioned and studied since (Reydon & Visser, 1967; Visser, 1968ab; Verhoeven & Van Vierssen, 1977, 1978). After removal of the ebb-

sluice during dike reconstructions in 1979, the last Dutchpopulation of Rissoa membranaceabecame

extinct (De Jong & De Kroon, 1982; Van der Goes et al. 1996; personal observations). In this I from note report on R. membranacea the Wadden Sea. Some hundred spec- imens collected shells from drift the Wadden Sea south of were as empty along coast the tidal marshes 'De in 1997. Also of shells Schorren', Texel, June a sample empty

from the inland brackish water 'de Bol', Texel, sieved from a bottom sample taken in

July 1997, was studied. In addition, samples of R. membranacea collected alive in the Wadden Sea between 1909 and 1929 (collection Zoological Museum Amsterdam,

ZMA) and from 'de Bol' collected in 1966 and 1968 (kindly provided by G.J.M. Visser,

Terschelling, now donated to ZMA) were studied. For comparison Recent specimens

from the province of Zeeland and from the Ria de Arosa (Spain) and fossil material

from Eemian deposits near Amsterdam were used (see table 1 for data on all samples used).

METHODS

Measurements ofthe top-whorl were made following Verduin (1982b) (d and Do, see The shells fig.la). were vertically positioned in sand and drawings were made of the

first whorl using a drawing tube on a Wild M5 stereo microscope and 50x magnifi-

cation, resulting in a magnification of 56.8x of the drawing. Do and d were measured

on these drawings. They have the advantage over direct micrometer measurements of

giving a visual record of the data. Verduin (1982b) estimated the accuracy of meas- of Do 0.003 and d 0.005 when in excellent urements at mm at mm, apices were

condition. He did not mention that measurements also depend on the accurate vertical position of the shell and the somewhat arbitrary location ofthe line along which d and

Do are measured. I measured d and Do ten times in one specimen, every time 92 BASTERIA, Vol. 61, No. 4-6, 1998

standard positioning the shell anew in a vertical position. This gave averages and

deviationofO.139 ± 0.009 mm for d and 0.248 ± 0.010 mm for Do. In general at least

ten measured table specimens were per sample (if available, see 1).

Rissoa membranacea the from above and definition ofd and Do Verduin Fig. 1. (J. Adams), (a) apex seen following

(1982a,b);(b) the ribbed form (drawingby J.P. Reydon, 1997, length6.4 mm); (c) the ribless form that occurred

in the brackish inland water ‘de Bol’ on Texel (drawing by J.P. Reydon, from Reydon & Visser, 1967, length

5.2 mm). Cadee: Rissoa membranaceafrom the Dutch Wadden Sea 93

RESULTS

The shells from the Wadden empty Sea collected among Hydrobia-rich shell driftalong

the high water line ofthe wadden coast of Texel S of'de Schorren' injune 1997, ranged

in size from to 3.3 7.1 mm (average 4.9 mm), which indicates that juvenile shells were

not The varied from smooth ribless shells present. sculpture to a clear presence of ribs 16 whorl. (fig. lb), some per Most specimens still had marks ofthe original colour. Only 9 specimens had a well preserved top-whorl for measurements of d and Do (see table The of 1). average d was 0.145 mm, and for Do 0.252 mm. Verduin (1982b) observed two clusters in his measurements, shells with the larger top-whorl with values of Do

+ 0.55d > 0.237 line in he to R. membranacea those with (see oblique fig. 2) assigned s.s.,

the smaller R. labiosa. Do + 0.55d for the 9 shells from Texel top-whorl to ranges from

0.319 to that all to 0.428, indicating these shells belong Verduin's R. membranacea s.s.

The data for the shells collected alive in the Wadden Sea between 1909 and 1926

(table 1 and fig. 2a) indicate that all had the of with values large type top-whorl average for d of between 0.153 and 0.171 and for Do of between mm 0.259 and 0.282 mm,

thus pointing to R. membranacea s.s. of Verduin (1982b).

The specimens from the inland brackish water 'de Bol' were all ribless (see fig. lc). The shells well the empty were preserved, top-whorl intact; apparendy the black mud rich in calcareous material such as shells and bryozoans is a good preserving medium. Measurements of these shells well empty as as from those collected alive here in 1966 all indicate the of the Wadden Sea 1 same large type top-whorl as specimens (table and fig. 2 b).

collector and d Do Locality year av. (s.d.) av. number coll.

measured pm (s.d.) pm

Texel W.C. v. Heurn 1909 153 (20) 261 (32) 9 ZMA

Den Helder H. Boschma 1915 161 (23) 264 (24) 12 ZMA

Den Helder H. Boschma 1915 163 (21) 276 (22) 4 ZMA

Den Helder 144 259 10 ZMA Vangdam T. v. Benthem Jutting 1919 (20) (31)

Den Helder T. v. Benthem 1919 171 oesterput Jutting (33) 282 (26) 10 ZMA

Waddenzee, Doove Balg T. v. Benthem Jutting 1926 164 (14) 282 (27) 10 ZMA

Texel, Schorren (empty) G.C. Cadée 1997 155 (22) 270 (25) 9 GCC

De Bol Texel C J.M. Visser 1966 170 (18) 280 (18) 10 ZMA

De Bol Texel (empty) CJ.M. Visser 1968 158 (17) 273 (21) 10 GCC

De Bol Texel (empty) H & G.C. Cadée 1997 145 (17) 252 (18) 21 GCC

Zeeland Nw Stjoosland C. Brakman 1922 161 (20) 274 (20) 10 ZMA

Zeeland Wemeldinge B. Hubert 1925 138 (8) 251 (7) 2 ZMA

Zeeland Kattendijke C. Brakman 1942 161 (2) 275 (36) 2 ZMA

Eemian Amsterdam (fossil) K.Jonges 1964-7 110 (17) 186 (28) 21 ZMA

G.C. Cadée 1960 Eemian Amsterdam (fossil) c. 122 (23) 226 (24) 11 GCC

Ria de Arosa G.C. Cadée 1964 104 (10) 180 (15) 15 GCC

Table ofRissoa membranacea shells collected alive when indicated otherwise. Columns 1. Samples measured, except

from left to collector and of and for d and right: sampling locality, year collection, average standard deviation

Do, number of specimens measured, and collection (ZMA = Zoological Museum Amsterdam, GCC = G.C.

Cadée). 94 BASTERIA, Vol. 61, No. 4-6, 1998

Verduin’s empty Eemian and from 1926 indicates shells and (c) graph 1909 1964. 1997; the in in between and alive line 1968 alive oblique in collected shells the collected Arosa a,b), empty de Ria (1982 specimens as and de both from Verduin 1966

alive following population, specimens Sea collected (d) sand; membranacea, Wadden specimens (a) dredged 1: Rissoa on for table from based Do also and Bol’ see collected d ‘de of labiosa, from R. Amsterdam dimensions and population near apical (b) of membranacea 1997; deposits R. Measurements his in separating collected 2.

Fig. line shells Cad.ee: Rissoa membranaceafrom the Dutch Wadden Sea 95

For taken of three from the of comparison measurements were samples province

Zeeland ofwhich only the sample ofNieuw en St. Joosland was large enough for reliable values The average (table 1). measurements of d and Do also fall in the range of the

Wadden Sea and 'de Bof specimens. We can conclude that all this material belongs

to R. membranacea s.s. Verduin (1982b).

The Dutch of R. membranacea in which I did find also the smaller only samples type of from the the apex were Eemian, averages for d and Do are clearly lower than for the Recent Dutch specimens (table 1, fig. 2d). Unfortunately in most specimens the apex was not sufficiently preserved for measurements. There is, however, no clear-cut difference between with the different of Verduin specimens types apex as suggested by (1982b), the transition is gradual, indicating the impossibility in the Eemian material

to his R. labiosa and R. membranacea the base of dimensions. distinguish on apical

For comparison I included a sample from the Ria de Arosa (table 1, fig. 2d). Apical

dimension appeared comparable to those ofthe Dutch Eemian sample and also showed

a gradual transition of lower values for d and Do to higher values, not separated in ‘R. labiosa’ and ‘R. membranacea’ cluster Verduin a a as suggested by (1982b, fig. 1). Even in Verduin's based material from in fig. 1, on Bretagne, one can see my opinion a transition in dimensions. Based gradual apical on measurements presented here, I

doubt whether both nominal species can be distinguished in the Ria de Vigo as Rolan (1983) did. For the nearby Ria de Arosa Cadée (1968) mentionedonly R. membranacea,

this is confirmed by the measurements given here.

DISCUSSION

The collected alive well specimens as as the empty shells from the Wadden Sea and

'de BoF Texel and also the collected alive in Zeeland all on two samples belong to type B of Rehfeldt (1968) and Waren (1996) with the large top-whorl indicative of lecitho-

trophic larvae without or with only a short period of pelagic life following Thorson Therefore (1950). I conclude that all Dutch Wadden Sea R. membranacea belonged to and the which Verduin the R. membranacea one same type, to (1982 a,b) gave name s.s.

I to the with a more slender (J. Adams, 1800). propose, however, not separate specimens of which found in the Ria de and Eemian top-whorl, some were Arosa samples, as a

different species R. labiosa (Montagu, 1803) as Verduin did (supported later by Bouchet,

1989), but to follow older authors (e.g. Jeffreys, 1867; Fretter & Graham, 1978) and

assemble all in one variable species R. membranacea s.l., because the transition from large

to slender apices is gradual (see the Ria de Arosa and Eemian samples, fig. 2cd). I also include in R. membranacea both A and B of Rehfeldt also propose to type (1968) as Waren did. with (1996) I agree Waren that Rissoa membranacea has to be seen as a species

with two different types of larval development, a phenomenon not common in gastro- 1950 few but in other pods (Thorson, gives a examples), more common groups e.g. polychaetes (Thorson, 1950). If the lecithotrophic larvae predominate in the northern of its and of part distribution, the planktotrophic in the southern part, the average size fossil the top in R. membranaceamight be a useful indicatorofpalaeotemperature. Bouchet

(1989) critically reviewed published data on poecilogony (variation in mode of develop-

ment) in prosobranchs; most reports were not documented well enough, but Bouchet

(1989) and Waren later (1996) do not exclude the possibility of poecilogony. Waren

to local conditions the of (1996) suggests adaptation to regulate presence planktotrophy

or lecithotrophy in R. membranacea. 96 BASTERIA, Vol. 61, Mo. 4-6, 1998

More than larval within - one development type a species (feeding non-feeding) is and from of with dilferentlarval relatively rare ranges interbreeding populations types

to of larvae of dilferent size from hatching a single egg mass (Havenhand, 1995). It has the of both larval maximal utilisationof local potential advantages types: resources and ability to colonise distant habitats. In the Roskilde fjord Rehfeldt observed both

and of larvae A and she pelagic non-pelagic development (her type type B), presents of the difference in larval shell did drawings size, and not mention intermediate spec- imens. Waren (personal communication, 1997) studied R. membranacea from the Roskilde

and other Danish localities and found no intermediate but either fjord specimens, type A with wide in the dimensions. Measurements of Verduin or B, a gap (1982b) for a sample from Bretagne and those presented here from de Ria de Arosa and the Eemian transition between both also sample, however, suggest a gradual types expressed in the dimensions of the larval shell. This casts doubt on Rehfeldt's statement (I.e.: 170) that dimensions of the initial shell embryonic are "undoubtedly genetically determined". The of occurrence only one type of larval development in the Dutch Wadden Sea viz. larvae with lecithotrophic no, or only a short, period of pelagic life, might have evolved here to minimise the loss of larvae from the eelgrass beds and thus illustrates

Waren's suggestion of the influence of local adaptation (Waren, 1996). Strong tidal and currents relatively short flushing times of the western Wadden Sea (now in the order of one to two weeks, will make return of Zimmerman, 1976) open water pelagic larvae to the eelgrass beds difficult. Thus lecithotrophy and non-pelagic larvae, or only a short the leaves described pelagic stage among Zostera as by Smidt (1938) and Thorson

(1946) will be an advantage. Moreover, in this short-lived species, where adults die after there will be food egg-laying, no competition between juveniles and adults of the same species, whereas such a possible competition in other longer-living species might favour dispersal by pelagic larvae.

ACKNOWLEDGMENTS

Many thanks are due to J.P. Reydon (Texel) for preparing fig. lb, to G.J.M. Visser for (Terschelling) providing material from 'de Bof on Texel, R.G. Moolenbeek and

R.H. de Bruyne (ZMA) for the loan of specimens, the latter also for his help and advice, and to Hans Cadee for her with in the field. I also for help sampling am very grateful critical remarks made by Basteria's referees and M.C. Cadee, R. Dekker, C. Swennen and A. Waren on an earlier draft of this paper, to HJ. Hoenselaar for help with

to literature and J.P. Reydon and Basteria for the use of fig. lc. This is N.I.O.Z. publication no. 3216.

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